Vanina Ruhlmann-Kleider DAPNIA/SPP (Saclay) V.Ruhlmann-KleiderPhysics at LHC, Praha Review of Higgs boson searches at LEP Introduction The SM Higgs boson Other scenarios Conclusions

V.Ruhlmann-KleiderPhysics at LHC, Praha The LEP environment: LEP1: ~17 millions of Z decays, e.g: LEP2: ~50 thousand of W + W - pairs a typical Higgs boson production cross-section

V.Ruhlmann-KleiderPhysics at LHC, Praha Search strategy: the SM Higgs as an example 1. Direct searches: sensitivity to low masses ≤O(100GeV) 2. Indirect constraints from precision measurements : sensitivity to low and high masses two complementary ways

V.Ruhlmann-KleiderPhysics at LHC, Praha Direct searches: experimental signatures production processes (+ WW, ZZ fusion) HiggsstrahlungPair-production Yukawa production SM, MSSM, 2HDM MSSM, 2HDM Only 3!

V.Ruhlmann-KleiderPhysics at LHC, Praha Clean experimental environment:  ~all final states investigated for masses up to O(100 GeV) precise models tested (SM, MSSM, 2HDM…)  model-independent results set as limits on  x Br Higgs boson decays Governed by masses more than by details of the model: main decays: into fermions also: in gauge or Higgs bosons  a limited nb of topologies to consider SM

V.Ruhlmann-KleiderPhysics at LHC, Praha Tools for the direct searches: b-tagging Impact parameters: c  ~ 400  m  R  ~ 20  m &  Rz ~ 40  m PV SV IP Secondary vertices: additional discrimination b quarks c quarks Performant b-tagging:  ~ 40% P ~ 90% at the jet level Data/simulation agreement : ~ 5%

V.Ruhlmann-KleiderPhysics at LHC, Praha L3 Hqq channel likelihood Tools: multidimensional analyses and kinematic fits To reach the best S/B discrimination: multidimensional analyses (NN, likelihood …) To improve on signal mass reconstructions: kinematic fits with E,p conservation and mass constraints (eg H channel

V.Ruhlmann-KleiderPhysics at LHC, Praha Ex: DELPHI, Hqq channel,  S = GeV, Tools: statistical interpretation of the results To make an unbiased and powerful statistical anaysis of the search results: Stop selections at a loose level Use rates and additional discriminant information (2d !) to test data compatibility with: tools: likelihood ratio test-statistics (-2LnQ) and confidence levels background + signal

V.Ruhlmann-KleiderPhysics at LHC, Praha M H > 60 GeV (95% CL) M H > GeV (95% CL) Search for the SM Higgs boson: results Fast rise of  as  S  M H +M Z  a few pb -1 enough to test M H above threshold e.g. july 2000: hypothesis M H =110 GeV is excluded LEP1 S/B >10 -5 LEP2 S/B >10 -3 LEP1 result:LEP2 result: final

V.Ruhlmann-KleiderPhysics at LHC, Praha expected for background only (mean, ±1  and ±2  bands ) expected minimum of -2lnQ for signal + bkg data data: consistent with a signal of mass in the range compatibility with the hypothesis of a background fluctuation: 9% compatibility with the hypothesis of a 115 GeV signal: 15% M H = 115 – 118 GeV The likelihood curve for the SM Higgs boson: final

V.Ruhlmann-KleiderPhysics at LHC, Praha Final state: e + e - HZ qq bb Comparing signal and background probabilities: ln(1+s/b) = jets of particles 2 b-jets Reconstructed mass: M H =  3 GeV One event consistent with the SM Higgs production

V.Ruhlmann-KleiderPhysics at LHC, Praha Summary about the SM Higgs boson: (status as of winter 2003) EW precise measurements: M H ≤ 211 GeV (95% CL) Direct searches: M H ≥ GeV (95% CL) M H  [115,118] GeV ?

V.Ruhlmann-KleiderPhysics at LHC, Praha One Higgs boson with non-standard properties the H  bb decay: M H  85 GeV: BR  /  SM  7% the H  decay: M H  110 GeV: BR  /  SM  20% final final 115. GeV114. GeV

V.Ruhlmann-KleiderPhysics at LHC, Praha One non-standard Higgs boson the hadronic final-state: M H  97 GeV: BR  /  SM  30% the invisible decay mode: M H  107 GeV: BR  /  SM  30% GeV114.4 GeV NB: mass range extended to lower m H in individual results (summer ‘01)

V.Ruhlmann-KleiderPhysics at LHC, Praha Nb: H  W W,ZZ decays also covered (L3, DELPHI …) One non-standard Higgs the H  final-state: M H  96 GeV: BR  /  SM  3% decay mode independent search:  /  SM  1 for M H  81 GeV GeV (summer ‘02) (published)

V.Ruhlmann-KleiderPhysics at LHC, Praha Combination of final results will also use more complete two-loop order radiative corrections: m h increased by a few GeV ! M h  91.0 GeV M A  91.9 GeV 0.5  tan   2.4 excl. (all limits at 95% CL) (summer ‘01) Neutral Higgs bosons in the CP-conserving MSSM m h max

V.Ruhlmann-KleiderPhysics at LHC, Praha No mixing 4GeV Neutral Higgs bosons in the CP-conserving MSSM (summer ‘01) M h  91.5 GeV (if tan  ≥0.7) M A  92.2 GeV (if tan  ≥0.7) 0.7  tan   10.5 excl. (all limits at 95% CL) hZ,hA+HZ (published) (summer ‘03) OPAL

V.Ruhlmann-KleiderPhysics at LHC, Praha minimal effect from CP phasesmaximal effect from CP phases Neutral Higgs bosons in the CP-violating MSSM OPAL: hZ and hA analyses reinterpreted to test MSSM scenarios with CP violation which allow for: more production processes (hence lower  ’s): H 1 Z, H 1 H 2 and H 2 Z H 2  H 1 H 1 beside the usual fermionic decays (winter ’03)

V.Ruhlmann-KleiderPhysics at LHC, Praha Yukawa production Neutral Higgs bosons in 2 Higgs Doublet Models masses and couplings no longer constrained as in the MSSM more final states to be expected and hence analysed, e.g.  More general analyses of LEP data to cover less constrained topologies than in the SM or MSSM is rare in the MSSM (M h ~ M A when cos(  -  ) is large) but allowed in 2HD models is negligible in the SM and experimentally excluded in the MSSM but possible in 2HD models cascade decay

V.Ruhlmann-KleiderPhysics at LHC, Praha HDM results: Yukawa production Upper bounds on enhancement factor of the h/A couplings to fermions wrt SM (published) bbA/h  bb  bbA/h  bbbb  A/h  (final)

V.Ruhlmann-KleiderPhysics at LHC, Praha HDM results: example 2 hA production: bbbb and  final states hA and hZ productions: h  AA cascade (final)  x Br reduction factor hA production: hadronic final state

V.Ruhlmann-KleiderPhysics at LHC, Praha assuming: Br(H ±  ) + Br(H ±  cs) = 1 Charged Higgs bosons in 2HDM model-independent results: limits on  xBr, e.g. M H   78.6 GeV (95% CL) (summer ‘01)

V.Ruhlmann-KleiderPhysics at LHC, Praha Charged Higgs bosons in 2HDM: the WA decays H   W  A decays open at high mass, requiring dedicated analyses. Done by OPAL and DELPHI for m A  12GeV Then, combining all searches in 2HDM of type I (WA decays dominate for tan  1) leads to: M H   76.7 GeV (95% CL) (summer ‘03)

V.Ruhlmann-KleiderPhysics at LHC, Praha Even more exotic: doubly charged Higgs bosons H  searched for through their purely leptonic decays, both in pair- and single productions (OPAL). Lifetime accounted for (DELPHI). M H   97.3 GeV for any h  (95%CL) M H   98.5 GeV for h ll >10 -7 (95%CL) OPAL (summer ‘02) (published)

V.Ruhlmann-KleiderPhysics at LHC, Praha Conclusions LEP did open the era of Higgs boson searches with both the direct searches and the precise EW measurements. SM Higgs boson: M H <211 GeV M H >114.4 GeV M H  [115,118] GeV ? Br(H ±  ) + Br(H ±  cs) = 1 On most subjects, final combinations still to come Many other scenarios were investigated to test precise models beyond the SM or/and produce model-independent results on  xBr